1. Field of the Invention
This invention relates generally to tractors for moving within boreholes, and specifically to a hydraulically powered tractor having electrically controlled motors that control tractor position, speed, thrust, and direction of travel by controlling fluid pressure acting on pressure-actuated valves.
2. Description of the Related Art
The art of drilling vertical, inclined, and horizontal boreholes plays an important role in many industries, such as the petroleum, mining, and communications industries. In the petroleum industry, for example, a typical oil well comprises a vertical borehole which is drilled by a rotary drill bit attached to the end of a drill string. The drill string is typically constructed of a series of connected links of drill pipe which extends between ground surface equipment and the drill bit. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string to the drill bit. The drilling fluid is used to cool and lubricate the bit, and to remove debris and rock chips from the borehole, which are created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole.
The method described above is commonly termed xe2x80x9crotary drillingxe2x80x9d or xe2x80x9cconventional drilling.xe2x80x9d Rotary drilling often requires drilling numerous boreholes to recover oil, gas, and mineral deposits. For example, drilling for oil usually includes drilling a vertical borehole until the petroleum reservoir is reached, often at great depth. Oil is then pumped from the reservoir to the ground surface. Once the oil is completely recovered from a first reservoir, it is typically necessary to drill a new vertical borehole from the ground surface to recover oil from a second reservoir near the first one. Often a large number of vertical boreholes must be drilled within a small area to recover oil from a plurality of nearby reservoirs. This requires a large investment of time and resources.
In order to recover oil from a plurality of nearby reservoirs without incurring the costs of drilling a large number of vertical boreholes from the surface, it is desirable to drill inclined or horizontal boreholes. In particular, it is desirable to initially drill vertically downward to a predetermined depth, and then to drill at an inclined angle therefrom to reach a desired target location. This allows oil to be recovered from a plurality of nearby underground locations while minimizing drilling. In addition to oil recovery, boreholes with a horizontal component may also be used for a variety of other purposes, such as coal exploration and the construction of pipelines and communications lines.
Two methods of drilling vertical, inclined, and horizontal boreholes are the aforementioned rotary drilling and coiled tubing drilling. In rotary drilling, a rigid drill string, consisting of a series of connected segments of drill pipe, is lowered from the ground surface using surface equipment such as a derrick and draw works. Attached to the lower end of the drill string is a bottom hole assembly, which may comprise a drill bit, drill collars, stabilizers, sensors, and a steering device. In one mode of use, the upper end of the drill string is connected to a rotary table or top drive system located at the ground surface. The top drive system rotates the drill string, the bottom hole assembly, and the drill bit, allowing the rotating drill bit to penetrate into the formation. In a vertically drilled hole, the drill bit is forced into the formation by the weight of the drill string and the bottom hole assembly. The weight on the drill bit can be varied by controlling the amount of support provided by the derrick to the drill string. This allows, for example, drilling into different types of formations and controlling the rate at which the borehole is drilled.
The inclination of the rotary-drilled borehole can be gradually altered by using known equipment, such as a downhole motor with an adjustable bent housing to create inclined and horizontal boreholes. Downhole motors with bent housings allow the ground surface operator to change drill bit orientation, for example, with pressure pulses from the surface pump. Typical rates of change of inclination of the drill string are relatively small, approximately 3 degrees per 100 feet of borehole depth. Hence, the drill string inclination can change from vertical to horizontal over a vertical distance of about 3000 feet. The ability of the substantially rigid drill string to turn is often too limited to reach desired locations within the earth. In addition, friction of the drilling assembly on the casing or open hole frequently limits the distance that can be achieved with this drilling method.
As mentioned above, another type of drilling is coiled tubing drilling. In coiled tubing drilling, the drill string is a non-rigid, generally compliant tube. The tubing is fed into the borehole by an injector assembly at the ground surface. The coiled tubing drill string can have specially designed drill collars located proximate the drill bit that apply weight to the drill bit to penetrate the formation. The drill string is not rotated. Instead, a downhole motor provides rotation to the bit. Because the coiled tubing is not rotated or not normally used to force the drill bit into the formation, the strength and stiffness of the coiled tubing is typically much less than that of the drill pipe used in comparable rotary drilling. Thus, the thickness of the coiled tubing is generally less than the drill pipe thickness used in rotary drilling, and the coiled tubing generally cannot withstand the same rotational, compression, and tension forces in comparison to the drill pipe used in rotary drilling.
One advantage of coiled tubing drilling over rotary drilling is the potential for greater flexibility of the drilling assembly, to permit sharper turns to more easily reach desired locations within the earth. The capability of a drilling tool to turn from vertical to horizontal depends upon the tool""s flexibility, strength, and the load which the tool is carrying. At higher loads, the tool has less capability to turn, due to friction between the borehole and the drill string and drilling assembly. Furthermore, as the angle of turning increases, it becomes more difficult to deliver weight on the drill bit. At loads of only 2000 pounds or less, existing coiled tubing tools, which are pushed through the hole by the gravity of weights, can turn as much as 90xc2x0 per 100 feet of travel but are typically capable of horizontal travel of only 2500 feet or less. In comparison, at loads up to 3000 pounds, existing rotary drilling tools, whose drill strings are thicker and more rigid than coiled tubing, can only turn as much as 30xc2x0-40xc2x0 per 100 feet of travel and are typically limited to horizontal distances of 5000-6000 feet. Again, such rotary tools are pushed through the hole by the gravity force of weights.
In both rotary and coiled tubing drilling, downhole tractors have been proposed to apply axial loads to the drill bit, bottom hole assembly, and drill string, and generally to move the entire drilling apparatus into and out of the borehole. The tractor may be designed to be secured between the lower end of the drill string and the upper end of the bottom hole assembly. The tractor may have anchors or grippers adapted to grip the borehole wall just proximal the drill bit. When the anchors grip the borehole, hydraulic power from the drilling fluid may be used to axially force the drill bit into the formation. The anchors may advantageously be slidably engaged with the tractor body, so that the drill bit, body, and drill string (collectively, the xe2x80x9cdrilling toolxe2x80x9d) can move axially into the formation while the anchors are gripping the borehole wall. The anchors serve to transmit axial and torsional loads from the tractor body to the borehole wall. One example of a downhole tractor is disclosed in allowed U.S. patent application Ser. No. 08/694,910 to Moore (xe2x80x9cMoore ""910xe2x80x9d). Moore ""910 teaches a highly effective tractor design as compared to existing alternatives.
It is known to have two or more sets of anchors (also referred to herein as xe2x80x9cgrippersxe2x80x9d) on the tractor, so that the tractor can move continuously within the borehole. For example, Moore ""910 discloses a tractor having two grippers. Longitudinal (unless otherwise indicated, the terms xe2x80x9clongitudinalxe2x80x9d and xe2x80x9caxialxe2x80x9d are herein used interchangeably and refer to the longitudinal axis of the tractor body) motion is achieved by powering the drilling tool forward with respect to a first gripper which is actuated (a xe2x80x9cpower strokexe2x80x9d), and simultaneously moving a retracted second gripper forward with respect to the drilling tool (xe2x80x9cresettingxe2x80x9d), for a subsequent power stroke. At the completion of the power stroke, the second gripper is actuated and the first gripper is retracted. Then, the drilling tool is powered forward while the second gripper is actuated, and the retracted first gripper is simultaneously reset for a subsequent power stroke. Thus, each gripper is operated in a cycle of actuation, power stroke, retraction, and reset, resulting in longitudinal motion of the drilling tool.
The power required for actuating the anchors, axially thrusting the drilling tool, and axially resetting the anchors may be provided by the drilling fluid. For example, in the tractor disclosed by Moore ""910, the grippers comprise inflatable engagement bladders. The Moore tractor uses hydraulic power from the drilling fluid to inflate and radially expand the bladders so that they grip the borehole walls. Hydraulic power is also used to power forward cylindrical pistons residing within propulsion cylinders slidably engaged with the tractor body. Each such cylinder is longitudinally fixed with respect to a bladder, and each piston is axially fixed with respect to the tractor body. When a bladder is inflated to grip the borehole, drilling fluid is directed to the proximal side of the piston in the cylinder that is secured to the inflated bladder, to power the piston forward with respect to the borehole. The forward hydraulic thrust on the piston results in forward thrust on the entire drilling tool. Further, hydraulic power is also used to reset each cylinder when its associated bladder is deflated, by directing drilling fluid to the distal side of the piston within the cylinder.
Tractors may employ a system of pressure-responsive valves for sequencing the distribution of hydraulic power to the tractor""s anchors, thrust, and reset sections. For example, the Moore ""910 tractor includes a number of pressure-responsive valves which shuttle between their various positions based upon the pressure of the drilling fluid in various locations of the tractor. In one configuration, a valve can be exposed on both sides to different fluid streams. The valve position depends on the relative pressures of the fluid streams. A higher pressure in a first stream exerts a greater force on the valve than a lower pressure in a second stream, forcing the valve to one extreme position. The valve moves to the other extreme position when the pressure in the second stream is greater than the pressure in the first stream. Another type of valve is spring-biased on one side and exposed to fluid on the other, so that the valve will be actuated against the spring only when the fluid pressure exceeds a threshold value. The Moore tractor uses both of these types of pressure-responsive valves.
It has also been proposed to use solenoid-controlled valves in tractors. In one configuration, solenoids electrically trigger the shuttling of the valves from one extreme position to another. Solenoid-controlled valves are not pressure-actuated. Instead, these valves are controlled by electrical signals sent from an electrical control system at the ground surface.
One limitation of prior art tractors is that they provide limited control over tractor position, speed, thrust capacity, and direction of travel. For example, while Moore ""910 teaches a highly effective design, the tractor tends to travel at high speeds, except when under a large load. Thus, there is a need for a tractor which provides enhanced control over tractor position, speed, thrust, and direction of travel.
Accordingly, it is a principle advantage of the present invention to overcome some or all of these limitations and to provide an improved downhole drilling tractor.
The present invention provides a tractor configured to push and/or pull a bottom hole assembly and drill string through a borehole. The tractor is preferably used in conjunction with a coiled tubing drill system. Advantageously, the tractor is capable of moving long distances horizontally, and provides enhanced control over position, speed, thrust, and direction of travel, compared to prior art tractors. In particular, the tractor includes motors that control the position, speed, thrust, and direction of travel of the tractor. The motors can be electrically controlled by electronic command signals transmitted from logic componentry located at ground surface or on the tractor itself.
One goal of the present invention is to provide enhanced control over position and speed of the tractor. Accordingly, the present invention provides a tractor having a throttle valve and load control valves, which provide varying degrees of control over tractor speed and position. Desirably, the throttle valve provides relatively rougher control, and the load-control valves provide relatively finer control. The throttle valve and load-control valves can be controlled by electronic command signals transmitted by electronic logic componentry on the tractor or at ground surface.
In one aspect, the present invention provides a tractor for moving within a borehole, comprising an elongated body, a gripper longitudinally movably engaged with the body, a flow channel, a chamber, and a pressure-regulator. The elongated body has at least one thrust-receiving portion, such as an annular piston. The gripper has an actuated position in which the gripper limits relative movement between the gripper and an inner surface of said borehole, and a retracted position in which the gripper permits substantially free relative movement between the gripper and the inner surface of the borehole. The flow channel extends to the thrust-receiving portion of the body and is configured to contain a first fluid flowing to the thrust-receiving portion. The chamber is configured to contain a second fluid. The pressure-regulator is configured to control the pressure of the second fluid in the chamber. The tractor is configured such that the pressure of the second fluid in the chamber controls the flowrate of the first fluid in the flow channel, as the first fluid flows to the thrust-receiving portion.
In one embodiment, the pressure-regulator comprises first and second valve portions. The second valve portion has a closed position and an open position. In the closed position, the second valve portion mates with the first valve portion to prevent the second fluid from flowing out of the chamber. In the open position, the second valve portion permits the second fluid to flow out of the chamber between the first valve portion and the second valve portion. The second valve portion is biased to its closed position by a closing force that is controllable to control the pressure of the second fluid inside the chamber. In another embodiment, the pressure-regulator further comprises a biasing means providing the closing force. In another embodiment, the first valve portion comprises an orifice in fluid communication with the chamber, and the second valve portion comprises a plug sized and configured to seal the orifice. In yet another embodiment, the biasing means comprises a spring. Also, a controller, such as a motor, is provided to control the closing force. The motor is configured to be controlled by electronic command signals.
In another aspect of the present invention, the size of a portion of the flow channel can be altered to control the thrust received by the thrust-receiving portion from the first fluid. This is due to the fact that as the size of the flow channel increases, so does the volume flowrate of the first fluid. In another aspect of the invention, the tractor further comprises a first valve movable to vary the size of the portion of the flow channel, wherein the thrust received by the thrust-receiving portion is controllable by moving the first valve. In another aspect, the first valve has a first position in which the flow channel is closed, and a second position in which the portion of the flow channel has a maximum size. The valve is movable so that the flow channel can have multiple sustainable sizes greater than zero. In another aspect, the tractor further comprises an additional biasing means, such as a spring, which exerts a spring force onto the first valve. The spring force tends to push the first valve to its first position, and increases as the first valve moves toward the second position. The first valve is in fluid communication with the chamber configured to contain the second fluid, so that the first valve is configured to receive a pressure force from the second fluid. The pressure force opposes the spring force and tends to force the first valve toward its second position. Desirably, the position of the first valve is controllable by controlling the pressure of the second fluid in the chamber.
In another aspect, the present invention provides a tractor for moving within a borehole, comprising an elongated body and a gripper longitudinally movably engaged with the body. The elongated body has a thrust-receiving portion, such as an annular piston. The gripper has an actuated position in which the gripper limits relative movement between the gripper and an inner surface of the borehole, and a retracted position in which the gripper permits substantially free relative movement between the gripper and the inner surface of the borehole. The tractor is configured such that longitudinal movement of the thrust-receiving portion in a first direction relative to the gripper can be opposed by a fluid pressure force. The fluid pressure force is controllable to at least partially control the position and speed of the thrust-receiving portion relative to the gripper.
In another aspect, the present invention provides a tractor for moving within a borehole, comprising an elongated body, a gripper longitudinally movably engaged with the body, a container longitudinally fixed with respect to the gripper and longitudinally movable with respect to the body, and a first valve. The elongated body has a thrust-receiving portion, such as a cylindrical piston. The gripper has an actuated position in which the gripper limits relative movement between the gripper and an inner surface of the borehole, and a retracted position in which the gripper permits substantially free relative movement between the gripper and the inner surface. The container contains the thrust-receiving portion. The first valve is configured to prevent a first fluid on a first side of the thrust-receiving portion from being displaced by the thrust-receiving portion when the first fluid is below a threshold pressure.
In one embodiment, the above-mentioned threshold pressure can be varied. Advantageously, the tractor further comprises a second valve configured to regulate the pressure of a second fluid exerting a pressure force on the first valve, wherein the threshold pressure can be controlled by controlling the second valve.
In another embodiment, the tractor further comprises a chamber configured to contain a second fluid, and a pressure-regulator controllable to control the pressure of the second fluid in the chamber. In yet another embodiment, the first valve comprises a first orifice and a flow-restrictor. The first orifice is configured to be in fluid communication with the container. The flow-restrictor has a first surface in fluid communication with the first side of the thrust-receiving portion, and a second surface in fluid communication with the chamber. The second surface generally opposes the first surface. The flow-restrictor has a closed position in which the flow-restrictor completely restricts fluid flow through the first orifice, and an open position in which the flow-restrictor permits fluid flow through the first orifice. The first surface of the flow-restrictor is configured to receive a first pressure force from the first fluid, the first pressure force tending to move the flow-restrictor to its open position. The second surface of the flow-restrictor is configured to receive a second pressure force from the second fluid, the second pressure force tending to move the flow-restrictor to its closed position.
In another embodiment, the flow-restrictor is biased toward its closed position by a biasing force and is configured to move toward its open position when the first pressure force exceeds the sum of the biasing force and the second pressure force. In another embodiment, the first valve further comprises a biasing means providing the biasing force.
In another embodiment, the pressure-regulator comprises a second orifice, a plug, and a spring. The second orifice is in fluid communication with the chamber. The plug has a closed position in which the plug prevents the second fluid from flowing out of the chamber through the second orifice, and an open position in which the plug permits the second fluid to flow out of the chamber through the second orifice. The spring exerts a closing force onto the plug which tends to maintain the plug in its closed position. Desirably, the closing force is controllable to control the pressure of the second fluid inside the chamber. In yet another embodiment, the second valve further comprises a motor controlling one of compression or extension of the spring so as to control the closing force. The motor is configured to be controlled by electronic command signals.
Another goal of the invention is to provide greater control over the direction of travel of the tractor. Accordingly, the present invention provides a tractor comprising an elongated body, a gripper substantially as described above, a fluid distribution system, a reverser valve, and a motor. The body has a thrust-receiving portion having a first surface configured to receive hydraulic thrust to propel the body in a first longitudinal direction, and a second surface configured to receive hydraulic thrust to propel the body in a second longitudinal direction generally opposite the first direction. The fluid distribution system is configured to provide hydraulic thrust to the first and second surfaces. The reverser valve has a first position in which the distribution system provides hydraulic thrust to the first surface, and a second position in which the distribution system provides hydraulic thrust to the second surface. The motor is configured to control the position of the reverser valve.
In one embodiment, the reverser valve is biased into its first position, and the tractor further comprises a chamber and a pressure-regulator. The chamber is in fluid communication with a surface of the reverser valve, and is configured to contain a first fluid. The pressure-regulator is configured to control the pressure of the first fluid in the chamber. In use, the pressure of the first fluid opposes the bias of the reverser valve. Advantageously, the motor controls the pressure-regulator. In yet another embodiment, the pressure-regulator comprises a pilot valve having a first position and a second position. In the first position, the pilot valve is configured to permit higher pressure fluid into the chamber, wherein the higher pressure fluid is configured to exert a pressure force onto the surface of the reverser valve to push the reverser valve to its second position. In the second position, the pilot valve permits the first fluid to flow out of the chamber so that the bias maintains the reverser valve in the first position. Advantageously, the motor controls the position of the pilot valve.
In another aspect, the present invention provides a tractor for moving within a borehole, comprising an elongated body, a first gripper, a second gripper, a first elongated container, a second elongated container, a fluid distribution system, a reverser valve, and a motor. The body has first and second thrust-receiving portions on an outer surface of the body. Each gripper is longitudinally movably engaged with the body and has an actuated position in which the gripper limits relative movement between the gripper and an inner surface of the borehole, and a retracted position in which the gripper permits substantially free relative movement between the gripper and the inner surface. The first container is longitudinally movably engaged on the body and longitudinally fixed with respect to the first gripper. The first container defines a first elongated space between the first container and the body, and encloses the first thrust-receiving portion such that the first thrust-receiving portion fluidly divides the first space into a first chamber and a second chamber. Similarly, the second container is longitudinally movably engaged on the body and longitudinally fixed with respect to the second gripper. The second container defines a second elongated space between the second container and the body, and encloses the second thrust-receiving portion such that the second thrust-receiving portion fluidly divides the second space into a third chamber and a fourth chamber.
The fluid distribution system is configured to distribute fluid to the first, second, third, and fourth chambers to propel the body longitudinally. The reverser valve is configured to control the direction of the tractor. The reverser valve has a first position in which the tractor moves in a first longitudinal direction according to a first cycle of steps comprising: actuating the first gripper; retracting the second gripper; supplying fluid to the first chamber to propel the body in the first direction; supplying fluid to the fourth chamber to propel the second container in the first direction, the second container being propelled with respect to the body; actuating the second gripper; retracting the first gripper; supplying fluid to the third chamber to propel the body in the first direction; and supplying fluid to the second chamber to propel the first container in the first direction, the first container being propelled with respect to the body.
The reverser valve also has a second position in which the tractor moves in a second longitudinal direction according to a second cycle of steps comprising: actuating the first gripper; retracting the second gripper; supplying fluid to the second chamber to propel the body in the second direction which is generally opposite the first direction; supplying fluid to the third chamber to propel the second container in the second direction, the second container being propelled with respect to the body; actuating the second gripper; retracting the first gripper; supplying fluid to the fourth chamber to propel the body in the second direction; and supplying fluid to the first chamber to propel the first container in the first direction, the first container being propelled with respect to the body. Advantageously, the motor is configured to control the position of the reverser valve.
Yet another goal of the present invention is to provide a tractor in which the grippers are inflatable, and in which the deflation rates can be finely controlled to facilitate faster subsequent inflation and, hence, tractor speed. Accordingly, in one embodiment at least one gripper is inflatable to move to its actuated position and deflatable to move to its retracted position. The tractor further comprises a gripper control valve configured to define a first flow orifice and a second flow orifice. The gripper control valve has a first position in which fluid is configured to flow through the first flow orifice to the gripper to inflate the gripper to its actuated position, and a second position in which fluid is configured to flow from the gripper through the second flow orifice to deflate the gripper to its retracted position. Advantageously, the gripper control valve is configured to vary the size of the second flow orifice so that the deflation rate can be finely controlled.
Yet another goal of the present invention is to provide a tractor in which the timing of the power strokes and reset strokes can be more precisely controlled. Accordingly, the present invention provides a tractor for moving within a borehole, comprising an elongated body, a gripper, first and second valves, and first, second, third, and fourth fluid chambers. The body has a thrust-receiving portion having a first surface and a second surface generally opposing the first surface. The gripper is longitudinally movably engaged with the body, and has an actuated position in which the gripper limits relative movement between the gripper and an inner surface of the borehole, and a retracted position in which the gripper permits substantially free relative movement between the gripper and the inner surface.
The first valve has a first position in which the first valve directs fluid to the first surface of the thrust-receiving portion, and a second position in which the first valve directs fluid to the second surface of the thrust-receiving portion. The first valve has a first end surface configured to receive a first fluid pressure force acting to push the first valve to the first position of the first valve. The first valve is configured to receive a first opposing force opposing the first fluid pressure force. The second valve has a first position in which the second valve permits fluid communication between the first chamber and the first end surface, and a second position in which the second valve permits fluid communication between the second chamber and the first end surface. The second valve has a second end surface in fluid communication with the third chamber, and is configured to receive a second fluid pressure force acting to push the second valve to the first position of the second valve. The second valve also has a third end surface in fluid communication with the fourth chamber. The third end surface is configured to receive a third fluid pressure force opposing the second fluid pressure force. Pressure variations in the first, second, and third chambers cause the first and second valves to cycle between their first and second positions. Advantageously, the fluid pressure in the fourth chamber is controllable to control the movement of the second valve.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.